Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "ansible: update dpdk, trex and samplevnf install"
  - ansible: update dpdk, trex and samplevnf install

    we now clone samplevnf from git repo
    made install_samplevnf role that takes as parameter VNF name
    added ubuntu_server_cloudimg_modify_samplevnfs playbook

    Change-Id: I5724e3b844b3137542a5c293516893a5c80f5c61
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 15f6586..afd31c1 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 15f6586bf0d2ed6b3f03dc3574a4dff5d9a3c64e
+Subproject commit afd31c1c34ef66f23e17bbdb2316e7f4bc1bf7ec